The present invention relates to seals in a gas turbine for supplementing the chordal hinge seals between turbine nozzle segments and a turbine nozzle support ring and particularly relates to supplementary seals for substantially minimizing or eliminating leakage losses past the chordal hinge seals.
In a gas turbine, hot gases of combustion flow from combustors through first-stage nozzles and buckets and through the nozzles and buckets of follow-on turbine stages. The first-stage nozzles typically include an annular array or assemblage of cast nozzle segments each containing one or more nozzle stator vanes per segment. Each first-stage nozzle segment also includes inner and outer band portions spaced radially from one another. Upon assembly of the nozzle segments, the stator vanes are circumferentially spaced from one another to form an annular array thereof between annular inner and outer bands. A nozzle retaining ring coupled to the outer band of the first-stage nozzles supports the first-stage nozzles in the gas flow path of the turbine. An annular nozzle support ring, preferably split at a horizontal midline, is engaged by the inner band and supports the first-stage nozzles against axial movement.
In an exemplary arrangement, eighteen cast segments are provided with two vanes per segment. The annular array of segments are sealed one to the other along adjoining circumferential edges by side seals. The side seals seal between a high pressure region radially inwardly of the inner band, i.e., compressor discharge air at high pressure, and the hot gases of combustion in the hot gas flow path which are at a lower pressure.
Chordal hinge seals are used to seal between the inner band of the first-stage nozzles and an axially facing surface of the nozzle support ring. Each chordal hinge seal includes an axial projection which extends linearly along a chordline of the inner band portion of each nozzle segment. Particularly, the chordal hinge seal extends along an inner rail of each segment and which rail extends radially inwardly of the inner band portion. The chordal hinge seal projection lies in sealing engagement with the axially opposite facing sealing surface of the nozzle support ring.
During operation and/or repair of the first-stage nozzle, it has been found that warpage can leave gaps between the chordal hinge seals and the sealing surface of the nozzle support ring. These gaps enable leakage flow past the chordal hinge seals from the high pressure area radially within the annular inner band into the hot gas flow path. That is, the chordal hinge seals are inadequate to prevent leakage flow as the chordal hinge seal projections lose contact with the sealing surface of the nozzle support ring. Consequently, there is a need for a supplemental seal at the interface of the first-stage nozzles and nozzle support ring to minimize or eliminate the leakage flow past the chordal hinge seals.
In accordance with a preferred embodiment of the present invention, there is provided a supplemental seal between the first-stage nozzles and the nozzle support ring which eliminates or minimizes leakage past the chordal hinge seals and which is readily and easily installed. In a preferred embodiment, the supplemental seal includes at least one and preferably two back-to-back sheet metal plates or shims wrapped in a woven metallic cloth attached to the inner rail of each nozzle segment. The supplemental seal extends from a groove formed in the radial inward facing surface of the inner rail to a distal margin which engages against a first surface of the nozzle support ring at a location radially inwardly of the chordal hinge seal. The distal margin is preloaded and it, together with the high pressure compressor discharge air in the high pressure region of the turbine, biases the cloth seal against the annular surface of the nozzle support ring.
Particularly, the proximal margins of the cloth seal and shims are secured, together with a support bracket, within a groove formed along the radial inward facing surface of the inner rail. Each seal segment extends linearly in a chord-wise direction relative to the turbine axis and is provided in lengths corresponding to the circumferential extent of each inner rail.
A mid-body portion of each seal projects generally axially along the radial inward facing surface of the inner rail toward the nozzle support ring. A distal margin turns in a generally radially inward direction to form a flexible distal leg or margin in sealing engagement with the sealing surface of the nozzle support ring. The shims in the leg of the distal margin of each seal are slit back from their distal edge to form a plurality of fingers. The fingers of one shim leg overlie the slits between the fingers of the other shim leg. That is, the slits of the shims are staggered in a chord-wise direction relative to one another such that the slits of each shim leg are effectively sealed by the fingers of the opposing shim leg. The slits afford flexibility to the sealing surface of the supplemental seal leg.
In a preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a turbine nozzle segment having at least one stator vane and a radially inwardly extending inner rail having a second surface in axial opposition to the first surface, a seal extending from a radial inward facing surface of the inner rail for sealing engagement against the first surface to form a seal therewith, the seal including a seal segment having at least a first plate, an overlay of a woven metallic cloth and a distal margin covered by the cloth and sealingly engaging the first surface and a bracket extending along the inner rail and at least a portion of one side of the seal segment to support the seal segment from the inner rail, the margin of the seal segment being flexible and preloaded to seal against the first surface.
In a further preferred embodiment according to the present invention, there is provided a turbine comprising a turbine nozzle support ring having a generally axially facing first surface, a plurality of turbine nozzle segments each having at least one stator vane and a radially inwardly extending inner rail having a second surface in axial opposition to the first surface, a seal comprised of a plurality of seal segments each extending from a radial inward facing surface of the inner rail for sealing engagement against the first surface to form a seal therewith, each seal segment including a first plate, an overlay of a woven metallic cloth and a distal margin covered by the cloth and engaging the first surface and a plurality of brackets extending along the inner rail and one side of the seal segments to support the seal segments from the inner rail, the margins of the seal segments being flexible and preloaded to seal against the first surface.